Wireless communication networks are composed of cells with at least one base station in each cell. Information such as voice communication or data is routed to a base station in the vicinity of a wireless transceiver, such as a mobile transceiver, and then transmitted by the base station to the transceiver in what is denominated the forward link. In turn, information is transmitted from the transceiver to a base station (in what is denominated the reverse link) and then relayed through the network to the intended recipient.
As compared to the forward link, the power available at a transceiver, such as a mobile transceiver, for transmission in the reverse link is generally much smaller—typically ten or more times less than the forward link. Therefore, in a reverse link there is a smaller power range available to produce an acceptable signal-to-noise ratio relative to the extant interference. Such interference emanates not only from the environment (e.g. from microwave ovens, automobiles, and other man made sources of interference), but also from the transmissions of other transceivers in adjacent cells and in the same cell. (Transmissions initiated in other cells are generally a weaker source of interference but are often not negligible.)
The level of total interference that is manageable for the lower power levels of the reverse link depends on the type of transmission, e.g. data or voice, and upon the promised quality of service. Voice transmission often is acceptable despite a significant error rate while data transmissions at such significant error rates would be unacceptable. Similarly, different customers require different classes of service. A bank electronically transferring money might well require transmissions with a relatively low received bit error rate compared to a typical communication subscriber.
In a CDMA wireless system conforming to present-day standards (IS-95 and subsequent) the bit error rate for a reverse link transmission is controlled by a protocol employing an inner and outer control loop. This dual loop controls the power of the reverse link signal by sending a control signal in the forward link telling the transceiver the intensity level at which it should transmit. The higher this indicated reverse loop transmission level, the lower the corresponding bit error rate such transmission incurs. However, the higher the transmission power the more rapidly the battery of the transceiver loses charge and the greater the interference effect of such higher power transmission on other reverse loop transmissions. Thus the goal of an inner/outer loop expedient is to maintain an acceptable communication quality for all subscribers in all service classes by appropriately controlling reverse loop transmission power for each such subscriber.
The outer loop sets the power target for the reverse loop transmission. To set this target the frame error rate for each subscriber's reverse loop transmission is monitored. That is, each frame (presently 192 bits every 20 ms) undergoes a parity check, and parity failure is equated with frame failure. An acceptable frame error rate level, e.g. 1 percent, is set for all subscribers or for classes of subscribers. If the measured frame error rate exceeds this set level for a subscriber's reverse loop transmission, the power target is raised. If an unacceptable frame error level is not reached, the target is lowered. However, the increment of increase is typically 100 times the increment of decrease for a targeted frame loss of about 1 percent. For more demanding frame error targets (0.1 or 0.01 percent) the difference between the increment of increase and increment of decrease is even greater. Upon an incremental change in power target, the inner loop then sends a control signal at intervals typically of between 1.25 ms and 10 ms (100 to 800 Hz) ordering, as required by the new target, an incremental increase or decrease in power of approximately 1 dB. The inner loop continues rapidly (relative to the speed of the outer loop interval) adjusting the control signal by, for example, 1 dB in response to the relatively slow outer loop measurement and corresponding target adjustment.
Such CDMA systems have performed reliably but have some limitations that are not totally desirable. Because the up increment of the outer loop is much larger than the down increment even for modest error targets, the transmission power employed is on average higher than needed for acceptable performance. A safety factor is ensured, but higher transmission powers lead concomitantly to increased interference at other transceivers. Additionally, the two loop approach does not adequately react to rapidly changing transmission events. For example, the relatively slow outer loop does not react to a rapid data transmission burst occurring in time periods of a few frames or less despite the potential for producing unacceptable error levels. Similarly the outer loop is unable to change targets at a rate sufficient to maintain acceptable error levels for an extended but rapidly changing signal power before the relatively slow outer loop initiates compensation. Longer term transmission also encounters difficulties. A mobile transceiver localized for a period of time such as at a red traffic light in a location with a line of sight transmission to the base station results in a strong signal being received. Thus the outer and inner loop work together to substantially lower the associated transmission power. If the subsequent movement of the transceiver (such as driving into an area with surrounding buildings) results in a sudden reduction in received signal power, the relative slow response of the outer loop yields an unacceptable bit error rate for an extended time while the extremely low transmission power is raised substantially. Additionally, as the vehicle moves, the likelihood of rapid fluctuations in received power increases with the previously discussed associated difficulties.
Therefore, the tendency to set transmission power at a level higher than essential for acceptable performance and rapid changes in transmission power in the reverse loop limits the efficiency of communication service. It would be quite advantageous to have an approach that provides better service despite rapid power changes without inducing an unacceptable increase in the average transmission power in the network. That is, a system that does not rely on an ability to track and adapt to varying signal-to-noise conditions to produce an acceptable frame error rate is an elusive goal.